1. Field of Application
The present invention relates to an electric power switching apparatus such as a DC-DC converter or DC-AC inverter, which performs periodic switching of high levels of current at a specific switching frequency.
2. Description of Related Art
With such an electric power switching apparatus, the output power contains various harmonic components at high frequencies that are respective integral multiples of the switching frequency. Some of these harmonic frequency components may fall within a frequency band of radio communication or radio broadcasting and so cause noise interference, so that it is necessary to provide a low-pass filter to exclude these frequency components from the output of the electric power switching apparatus.
In particular, the switching frequency of a DC-DC converter may have a value of approximately 100 kHz, and when such a DC-DC converter is mounted in a vehicle at a location relatively close to a radio of the vehicle, it has been necessary to utilize a large-scale filter in order to reduce the radio noise interference to a sufficient extent. However such a filter occupies a large amount of space, and is expensive to manufacture.
In various countries, respective values of transmission carrier frequency are allocated to different radio broadcasting stations with a fixed amount of separation between adjacent carrier frequencies, to ensure that there will be a specific frequency gap that separates respective transmission frequency bands of radio stations having adjacent values of carrier frequency (with each transmission frequency band extending on either side of the corresponding carrier frequency).
In the case of AM (amplitude modulation) radio broadcasting, a separation of 9 kHz is established in Japan, for example, while the separation is 10 kHz in the U.S.A. The respective transmission carrier frequencies allocated to various radio stations will be referred to in the following as allocated carrier frequencies. Methods have been proposed for reducing the above-described radio noise interference, based on using the frequency gaps between the transmission frequency bands of respective radio stations, for example in Japanese patent publication No. 2002-335672, and in Japanese patent publication No. 2003-88101. These methods attempt to ensure, as far as possible, that no high-frequency components of the switching frequency of the electric power switching apparatus will overlap with any of the transmission frequency bands used by AM radio stations.
However such methods necessarily involve close control of the switching frequency. The narrower the gap between the respective wavebands of frequency-adjacent radio stations, the greater becomes the requisite accuracy of frequency control. These frequency gaps are narrow, e.g., 9 kHz or 10 kHz as described above. Due to that fact, and due to the inevitable changes which occur in frequency-determining components as a result of ambient temperature variations, manufacturing variations, long-term changes in component characteristics, etc., it is difficult to implement such methods on a practical basis.